Why is the sky *any* color?

A nice video from PBS that explains why the sky has color.

Why is the sky blue? It's a question that you'd think kids have been asking for thousands of years, but it might not be that old at all. The ancient Greek poet Homer never used a word for blue in The Odyssey or The Iliad, because blue is one of the last colors that cultures pick out a word for.

In this episode, I'll tell you not only why the sky is blue, but why it's red at sunset. It turns out, those colors are all part of the same sunbeam. And when you're looking at a blue sky, you could be sharing a special moment with someone thousands of miles away. Next time a kid (or the kid inside you) wants to know why the sky is blue, you'll have science to back you up!


  1. Are our eyes evolved to see unfiltered sunlight as “white” then? I have to wonder what our color palette might look like to eyes that evolved under a different star type.

    1.  The peak intensity is for green light. So some being that evolved under a red star would probably think of the earth as Emerald City.

      1. Sunlight doesn’t really have a “color” and just contains all the frequencies. So our eyes could have envolved to be sensitive to any of the colors. But green must have proven to be the most useful. 

        A red star would lack the green and blue frequencies, so the aliens probably wouldn’t have photoreceptor cells reacting to those frequencies. And be very good at seing red. 

        Also remember that a single frequency usually stimulates all 3 kind of cones and the color we see is constructed from the identities. And the same color can be seen by a different mix of frequencies that stimulate the cones the same way. 

          1. LOL.  Too bad they missed it out when Predator fought Schwarzenegger.  The predator should have been ‘blind’ after it took off its face shield.   

    1. answer; It is actually partly violet, your brain’s just weird and perceives a mix of blue and violet light as blue mixed with white.

      1. I was about to say there’s no such thing as violet light as such but then I looked it up. Either they put me wrong in design class or I’m just thinking of indigo. Seems there’s a twitch in the response of the red receptors down that end or something, hence the purplish impression?

        I’d assumed no-violet was what xkcd meant because there’s another example of a misleading question in the rollover text https://xkcd.com/1145/

        That said, Wikipedia giving RGB values for Violet then asserting it’s not the same a purple isn’t calculated to reassure the confused. (Of course, I’m not going to satisfy myself about this by looking at a book or a computer screen.)

        Anyway: NASA’s answer to the sky question here http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html#violet

    1. More importantly, Homer doesn’t describe things in colours, he uses words – including what we recognise as colours – to describe things in qualities. The sky is “bronze” not because it’s brown or tan but because it’s shining and bright; the sea is “wine-dark” because it’s deep and opaque.

      He’s a poet, he’s not going to write “the sky was fucking blue that day man, I swear – REALLY FUCKING BLUE”. His audience know what -colour- the sky is, he wants to evoke what it -looks like-.

  2. I’m not sure if I’d call his explanation wrong, but after seing the video, I wouldn’t have understood it. You might have noticed that if light travels a medium that scatters blue, it will become less blue, not more. Here is why the sky is blue after all: 

    The light traveling from the sun directly to your eyes (the sun itself) is still white. Without scattering, the rest of the sky would be black. But since the air bends the blue light rays, they can still reach earth. 

    The sunset is red mostly because the light is affected by larger particles (clouds, pollution) which scatter more of the red spectrum.

    1. I guess that you’ll appreciate this demonstration better :

      “You might have noticed that if light travels a medium that scatters blue, it will become less blue,”… while the medium itself appears bluer.

        1. Mommy mommy, the layer of gas sitting above us, why is it blue instead of white?

          (Air is a blue-colored material.  “Sky” is an illusory substance.   It can’t have a color.)

  3. The sky doesn’t have any color, it just looks like it does from where you are looking at it. The sky never changes color, it is the sunlight that does all that.

  4. Whenever my kids have asked why the sky is blue or red, I tell them “Rayleigh scattering.”  Best delivered in a deadpan voice without explanation.

    They don’t ask me many questions anymore.

  5. blue is one of the last colors that cultures pick out a word for

    I hope this item of Fail does not become the new colour-linguistics “Eskimo Words for Snow”.

    1. There’s actually a lot of work out there on how different languages and cultures handle color.  Some languages don’t distinguish as many colors as others do, and they may delimit those colors differently.  For instance, in the Scandinavian languages, the fruit we call an “orange” has a color they call “red”.

      All of that’s independent of what the paint-sellers and crayon-sellers call colors in those cultures, of course :-)

      1. Yes.  Irish has the word “glas” for natural green and gray.  There’s another green for non-natural objects, “uaine”.

  6. If you shine white light on a single nitrogen molecule, and view it against a dark background, it should look like a tiny sky-blue speck.  (Too small to see?   Nah, just use much brighter light!)

    So, if we hold a single nitrogen molecule up against a bright white background (e.g. light bulb,) it should appear as a dark red-orange shadow; a blotch the color of sunsets.

  7. Speaking of color and language, I’ve read a summary of a study that determined that native speakers of Russian are able to discriminate shades of blue more finely than non-Russian speakers.  Apparently the habitual use of that language also affects a part of the brain dedicated to color perception.  The brain is weird.

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